Research On Acoustic Oscillation Over The Duct Cavity And Suppression With Piezoelectric Vibrator

Currently, cavity flow problem is a hot research issue in the field of the aerodynamics. Firstly, cavity flow is the problem that is often encountered in aerospace applications with high worthy of engineering research. For example, weapon bays and wheel wells of aircrafts is able to cause severe flow-induced oscillation under high speed air flow. Besides, low speed airflow or water flow is also able to generate flow-induced oscillation and acoustic radiation. Furthermore, there are many typical theoretical issues in cavity flow problem, including unsteady flow, fluid dynamic instability and acoustics-vortex interaction, so it is of theoretical research value. Therefore, study on the mechanism of the cavity flow, prediction method the characteristic parameters and noise suppression of the cavity oscillation is of not only the value of engineering application but also the theoretical research.In this dissertation, based on the experiment with hot wire anemometer and dynamic pressure transducer, study on the unsteady flow of the self-oscillation of the cavity flow shear layer is carried out. Rules on disturbance to the shear layer and effect of acoustic mode on and the vortex structure at the cave mouth are investigated. The coupled relationship between the mode of the acoustic standing wave and the self-oscillation of the cavity flow shear layer is focused. Method of proper orthogonal decomposition which is used for turbulent coherent structures is extended in this dissertation, and is applied to fluctuation pressure field analysis. And space mode of the proper orthogonal decomposition to the fluctuation of the wall pressure under coupled condition between the mode of the acoustic standing wave and the self-oscillation of the shear layer. Piezoelectric cantilever vibrator is designed according to the study to its static and dynamic displacement response and natural frequency.Firstly, a cavity flow experimental platform with acoustic wind tunnel of low turbulence is established. A new structure intergrading capability of diffusion and muffling is designed, which solves the problem of noise elimination and flow loss reduction. With the wind tunnel, experiment for wall dynamic pressure measure and shear layer status is performed. Structure and flow speed are examined. Rules of the spectrum characteristics of the self-oscillation of the cavity flow shear layer are summarized and mechanism of the cavity oscillation is discussed. Prediction equation is improved for the frequency of self-oscillation of the cavity flow shear layer, and a prediction equation for low-speed flow of cavity flow oscillation is developed with high accuracy.Furthermore, vortex characteristic of the shear layer is investigated with discussion of the following factors of the shear layer: mean velocity distribution, thickness of the shear layer and velocity fluctuation. Based on the perdition of the frequency of the acoustic mode of the cavity oscillation, a model is developed that coupling the mode of the acoustic standing wave and the self-oscillation of the cavity flow shear layerThis dissertation extends the method of proper orthogonal decomposition which is used for turbulent coherent structures. Information of both domain of temporal-spatial and frequency-spatial are deduced and proper orthogonal decomposition is done to both time and frequency domain. Space mode of the proper orthogonal decomposition to the fluctuation of the wall pressure is obtained under coupled condition between the mode of the acoustic standing wave and the self-oscillation of the shear layer. It shows that under resonance condition, the intrinsic first mode expresses the periodic acoustic mode. Beside, the convergence of the intrinsic proper orthogonal decomposition of frequency domain is faster than that of the time domain, and pressure in formation with the reconstructed lower mode under frequency mode is more than that of the time domain.Lastly, based on the established prediction model of frequency, method of the cavity oscillation suppression method is explored. Both analytical and finite element model are established to study the dynamic and static response and resonance frequency of the piezoelectric cantilever vibrator. Structure of the double crystal piezoelectric is designed and experiment of the cavity oscillation suppression is performed. Results show that the piezoelectric beam is able to reduce the summit value of the pressure fluctuation. Furthermore, influence of the factors such as beam geometry and driving frequency on the effect of oscillation suppression are obtained.